Fou is a squirrel-like creature, "a cute animal" that appears to be able to freely walk around the Chaldea Security Organization. It is met by the Fujimaru Ritsuka while it is alongside Mashu.

Merlin uses Cath Palug to temporarily reveal himself in order to help the Protagonist against Cú Chulainn (Alter) in E Pluribus Unum Chapter. After stating that they should be able to meet in a slightly different future should the Grand Order project succeed, he disappears.

Fou is the 4th Beast among the 7 beasts in Fate/Grand Order. It sacrificed its own power as BEAST IV to revive Mashu Kyrielight, giving her a normal human's lifespan. After that, it became a normal animal beast. (These are all done while its own power isn't fully manifested yet as it is still not at its prime.)

https://fategrandorder.fandom.com/wiki/Fou